Wide angle horn feed closely spaced to main reflector



Sept. 20, 1966 A. F. KAY

WIDE ANGLE HORN-FEED CLOSELY SPACED TO MAIN REFLECTOR Filed April 3, 1965 5 Sheets$heet 1 RECEIVER TRANSMITTER FIG. 2

IN VEN TOR.

ALAN F. KAY BY J Q ATTORNEY A. F. KAY

Sept. 20, 1966 WIDE ANGLE HORN-FEED CLOSELY SPACED TO MAIN REFLECTOR 5 Sheets-Sheet Filed April 5, 1963 INVENTOR. ALAN E KAY BY fi /Lyaiv/Lwz ATTORNEY Sept. 20, 1966 A. F. KAY 3,274,503

WIDE ANGLE HORN-FEED CLOSELY SPACED TO MAIN REFLECTOR Filed April 5, 1963 5 Sheets-Sheet 3 INVENTOR.

ALAN F. KAY

ATTORNEY Sept. 20, 1966 A. F. KAY 3,274,603

WIDE ANGLE HORN-FEED CLOSELY SPACED TO MAIN REFLECTOR Filed April 3, 1963 5 Sheets-Sheet 4 INVENTOR.

I ALAN F. KQY BY [m1 0.4M

ATTORNEY Sept. 20, 1966 A. F. KAY 3,274,603

WIDE ANGLE HORN-FEED CLOSELY SPACED TO MAIN REFLECTOR Filed April 5, 1965 5 Sheets-Sheet 5 INVENTOR.

ALAN F. KAY BY [cumf 4. @454 ATTORNEY United States Patent O 3,274,603 WIDE ANGLE HORN FEED CLOSELY SPACED TO MAIN REFLECTGR Alan F. Kay, Cambridge, Mass., assignor, by mesne assignments, to Control Data Corporation, South Minneapolis, Minn., a corporation of Minnesota Filed Apr. 3, 1963, Ser. No. 270,330 g The portion of the term of the patent subsequent to Nov. 2, 1982, has been disclaimed and dedicated to the Public 7 Claims. (Cl. 343-781) This is a continuation-in-part of application, Ser. No. 230,802, filed Oct. 12, 1962, now Patent No. 3,216,018.

The present invention relates to wave translation systems and devices. More particularly, the invention relates to directive microwave antennas for transmitting and receiving microwave energy. More especially, the invention relates to microwave antennas having a primary radiator and curved reflector, such as a paraboloiclal reflector, or lens.

As used herein the term wave translation includes both the reception and transmission of radiated energy.

In the prior art it is well-known to employ microwave systems involving a paraboloidal reflector and primary feed radiator. In general, the size of the feed aperture and direction of the primary energy is determined so that all of the primary energy radiated strikes the reflector. In many large antennas which are located relatively close to a ground plane, the problem of high input receiver noise is aggrevated by spillover from the reflector or lens.

For the prior art antennas, the diameter of the primary radiator aperture is chosen with respect to the diameter and focal length of the reflector and the wavelength of the microwave energy. The primary radiator or feed horn is typically formed from a wave guide transmission line termination and frequently requires only a small taper in the E plane dimension of the guide and little if any taper in the H plane dimension to provide an aperture of proper size. Where a taper is required it is introduced sufficiently gradually so that the phase errors are minimized. In general, the true phase center of the feed appears in the aperture of the feed. Consequently, the focal point of the reflector is located at or slightly behind the feed aperture.

With such a design, a taper in the illumination of the reflector aperture is obtained from 1020 db, and the energy radiated by the primary radiator which fails to strike the reflector spillover energy is of the order of 1020% of the total energy radiated by the primary radiator. This spillover energy is not focused and is accordingly wasted.

For applications involving a sensitive low-noise receiver, the noise figure is limited primarily by the effective noise temperature of the antenna. In this circumstance the spillover energy causes an appreciable increase in the effective antenna noise temperature and a decrease in sensitivity. In the range, for example, of 1-10 kilomegacycles, a large paraboloidal reflector antenna pointed away from the horizon, neglecting the signals of radio sources, should see a cold sky temperature of 510 Kelvin.

With prior art primary feed structure, the spillover characteristic causes the antenna to pick up the noise of the warm earth, 270 Kelvin; thus it increases the effective noise temperature at the antenna to 20-40 Kelvin.

A further disadvantage of conventional horn primary feed systems involves the variation in beamwidth directly 3 ,274,603 Patented Sept. 20, 1966 with the wavelength. This results in degraded performance at one or both ends of the frequency spectrum. At the low frequency end of the band, the beam tends to be too broad and the spillover energy too large. At the high frequency end of the band, the beam tends to be too narrow. While the spillover at the high frequency end is small, the illumination taper of the reflector is so great that its gain is decreased, its bandwidth decreased and its resolution degraded.

Another disadvantage of the prior art feed systems arises in monopulse radar tracking systems. This is particularly true where a cluster of two to four horns are centered on the focal point. If the aperture of each horn is chosen small enough to provide a satisfactory illumination taper and spillover for the reflector for the sum mode of operation, the difference mode pattern produced is too broad. Conversely if the feed horn diameters are sufliciently large for the proper behavior of the difference mode, the illumination taper in the sum mode is too great.

Still another disadvantage of the conventional feed systems appears when a spherical reflector is utilized as, for example, for wide-angle performance requirements. In this circumstance large phase errors are introduced at the feed aperture.

In a primary radiator useful in the present invention it is highly desirable for the E plane and H plane radiation patterns to be balanced, that is, to have a similar illumination characteristic particularly at the aperture. This has the result of providing relatively low illumination at the edges of the aperture of the element. In order to accomplish this result, it is highly desirable to provide an effective electrical ground plane or impenetrable wall operative for both the electric and magnetic fields. It is well known that a plane surface provides an effective wall for the electric field. The tangential component of the electric field E vanishes on the surface of a good conductor. However, no known natural surface has the same property for the magnetic field H. In order to produce the result of a compatible electric and magnetic field illumination, it is highly desirable to provide a surface having similar properties with respect to the magnetic field H as a conductive surface provides for the electric field E.

It is therefore a primary object of the present invention to provide an improved wave translation element for efficient reception and transmission of microwave energy.

A further object of the invention is to provide an improved wave translation element useful as a primary radiator for a microwave antenna compatible with we tremely low noise receivers.

Another object of the invention is to provide an improved wave translation element exhibiting low illumination at the edges of an illumination aperture.

Still another object of the invention is to provide an improved wave translation element for use in a high gain microwave antenna of substantially improved efficiency.

In accordance with the invention there is provided a wave translation element. The element includes a member having a wave translation surface. Radiation suppression means are coupled to the surface. The suppression means include a groove formed in the member. Means are provided for coupling the member to a source of electro-magnetic energy.

In one embodiment of the invention, the member has a circularly bounded, angle-defining wave translation aperture. It includes a conical wave translation surface for producing a substantially spherical wave front at the aperture. Radiation suppression means are coupled to along the depth d of the groove.

the surface. Suppression means include a plurality of uniformly spaced grooves formed in the member throughout the surface. The width of each groove is greater than one-quarter wavelength. The spacing between the grooves is less than one-half wavelength. The depth of each groove is between approximately a quarter and a half wavelength at an operating frequency. Means are provided for coupling the member to a source of electromagnetic energy.

Other and further objects of the invention will be apparent from the following description of the invention, taken in connection with the accompanying drawings and its scope will be pointed out in the appended claims.

In the drawings:

FIG. 1 is a plan schematic view of a microwave antenna useful with the present invention;

FIG. 2 is a front view of the antenna portion of FIG. 1;

FIG. 3 is a side sectional view of the primary feed horn for the antenna in FIG. 1;

FIG. 4 is a side sectional view of a modification of the horn in FIG. 2 and embodying the invention.

FIG. 5 is a front view of the horn in FIG. 4;

FIG. 6 is a side sectional view of a further modification of the horn in FIG. 3 and embodying the invention;

FIG. 7 is a front view of the horn in FIG. 5;

FIG. 8 is a side sectional view of another modification of the horn in FIG. 2 and embodying the invention;

FIG. 9 is a front view of the horn in FIG. 8;

FIG. 10 is an enlarged detailed view of a section of the horn in FIG. 8;

FIG. 11 is a perspective view of a primary radiator illustrating a modification of the horn in FIG. 8; and

FIG. 12 is a front view of the feed horn of FIG. 11.

PRINCIPLES OF OPERATION In my copending application noted above, the principles of ope-ration of an antenna system, in which the primary radiator of the present invention is particularly useful, are disclosed. In that application, the broad principles of a focusing wave translation device are discussed.

Here we are particularly concerned with a wave translation element having a propagating or wave translation surface for electromagnetic waves. The surface presents a perfect wall to tangential components of both the magnetic and electric fields.

It has been found that a radiation suppression means may be coupled to the surface for balancing the electric and magnetic fields to provide a desired illumination characteristic.

Equation 1 x on surface 'where E is the electric field component along the direction of travel, H is the magnetic field component along the groove, 6 is the impedance of free space, j= /1 Yand k=211-/)\ where A is a wavelength.

For d an odd multiple of quarter wavelengths, Z=oo and Hy=0, where Hy is themagnetic field component The surface thus provides a magnetic wall for waves traveling normal or transverse to the grooves. Equation 1 is valid for w=g+t= center-to-center spacing between grooves less than one half wave length. Since the groove width g is preferably greater than the distance 2 between grooves, t is prefer-ably less than a quarter wavelength.

The electric fie'ld component E along the length of a groove is also zero. The surf-ace presents a wall to both electric and magnetic fields; i.e. E =Hy=O.

It can be shown that the surface has positive or inductive reactance if:

Equation 2 where n is an integer. Such a surface supports a surface wave. For a wave traveling across a groove, a surface wave would be excited of relatively high field strength.

However, it turns out that the surface presents a wall for both magnetic and electric fields, i.e., fails to support surface waves if:

Equation 3 (n+ /2)1r kd (n+1)1r for n=0, 1, 2, etc.

Such a surface provides a capacitive reactance. In this condition the fields shy away from the surface and the surface indeed presents a Wall.

While Equations 2 and 3 are approximate, there appears to be a sharp cut-off of excitation or propagation of surface wave at kd slightly less than (n+ /2)1r. For the case in Equation 3 in which n=0, and d is between a quarter and a half wavelength, a wall for both electric and magnetic fields is presented effective for a substantial band of frequencies. A bandwidth of at least 2 to 1 is readily available in practice. 7

Such a wall is useful for wave translation elements. For waveguide transmission lines, e.g., such a wall presents the same impedance boundary conditions to both electric and magnetic polarizations. If the guide has two planes of symmetry, such as square or circular guide, the transverse field distribution is the same in both principal planes of polarization. Such a waveguide, adapted to provide primary radiation for an antenna system, produces radiation patterns identical for both E and H planes. Typically such a guide may be coupled to a flared primary radiator or feed horn.

Optimum gain and side lobe levels may be realized with a primary radiator embodying the invention. A dual or circularly polarized radiator with proper illumination taper in both the E and H planes is made possible.

In one form of the invention a primary radiator having a wide angle flared member with elongated grooves extending transversely to the propagating surface has been successfully demonstrated. The grooves are uniformly spaced and extend throughout the surface parallel to the transverse planes relative to the axis of, e.g., a reflector. A waveguide transmission line is coupled to the member. The waveguide has no grooves. The first groove is disposed less than approximately M10 from the junction between the waveguide and the member.

Such a radiator eliminates spurious sources of radiation at the feed aperture near or at the edges in the E plane. The E and H plane beam widths are maintained equal throughout the frequency bandwidth of the Waveguide.

Only a desired dominant mode is excited in the flared member. The impedance of the member may be matched to that of the waveguide by tapered sections, quarter wave transformers, dual mode transducers or circular polarizers. The latter two take full advantage of the dual polarization capability of a radiator embodying the invention.

DESCRIPTION AND EXPLANATION OF THE ANTENNA IN FIG. 1

Referring now to the drawings and with particular reference to FIG. 1, there is here illustrated a microwave antenna system embodying the invention. The antenna is generally indicated at 20. A primary wave translation 5 means or radiator 21 provides the illumination for a focus ing wave translation means, a paraboloidal reflector 22. A receiver-transmitter 23 is coupled through a wave guide section 24 to the radiator 21. The antenna as shown has an axis of propagation 25. The primary radiator 21 includes an angle-defining member which is a conically shaped feed horn having a vertex substantially co-incident with the focal point 26 of the paraboloid 22. The focal distance from the center of the paraboloid to the focal point 26 is indicated as F. A bounded illumination aperture is defined by the circular aperture of the reflector 22. The diameter of the reflector at the plane is indicated as D. The angle of illumination defined by the circular boundary of the illumination aperture and the focal point is indicated as Half the flare angle of the feed horn 21 is indicated as Half the angle (,0 is indicated as 0 and the angle of a generalized ray of energy is 0 relative to the propagation axis 25. The sector of illumination involving a phase error is indicated as 6.

It will be apparent that: Equation 4 and that a phase error exists in the region where: Equation One half the aperture width is indicated as a; the entire feed aperture width or diameter being 2a or D The feed horn length is indicated as r The dashed lines 27 and 28 indicate the boundary, extended to infinity, of the sides of the flared feed horn 2 1. The dashed lines 29 and 30 indicate the boundaries of illumination angle The plane of ground or the earth is indicated at 31.

In FIG. 2 there is a front view presented of the antenna portion of the schematic diagram in FIG. 1.

Energy is received as captured by 'thc'r'eflector 22 and directed toward the horn 21 which is coupled through a wave guide transmission line 24 to the receiver 23. Conversely, transmitted energy may be coupled through the guide 24 to the feed horn 21, directed to the reflector 22 and radiated into space along the axis of propagation 25.

side sectional view of a conical primary radiation means. A conical angle-defining feed horn member 32 is coupled to a section of rectangular wave guide transmission line 33. Here the focal point of a reflector is shown at 34 in the vicinity of the junction of the throat and the flared section of the feed horn 32. A transition taper indicated at 35 is introduced to provide impedance matching be effect.

surfaces of the feed horn 32 into the illumination angle. The elements as shown are perpendicular to the inside conical surface and aligned in the E plane as indicated in the front view of FIG. 5.

The introduction of such blocking obstacles as the rodlike elements 36 and 37 produces an illumination of the horn aperture in theE plane which is then similar to that of the H plane, having relatively low illumination of the edges. The radiation patterns in both planes then become compatible. The height, diameter and disposition of the elements 36 and 37 are so selected as to reduce the excessive E plane radiation to a level commensurate with the H plane radiation to reduce excessive illumination of the edges.

Referring now to FIGS. 6 and 7, there is here illustrated a modification of the radiation suppression means described with respect to FIGS. 4 and 5. Here the rods are replaced by an annular member 38. By replacing the rods as shown in FIGS. 4 and 5 with a thin metal annulus, the E plane pattern behavior is affected as indicated above with respect to the rods. It turns out that the annulus has very little effect on the H plane pattern. The annulus preserves axial symmetry so that a dual polarized wave guide transmission line may be used leading into the flared horn with equal performance for both polarizations because of symmetry. The aperture of the annulus is chosen to be large enough to avoid cutoff of the dominent spherical wave TE mode and small enough to excite a suflicient amount of the TE mode substantially to eliminate illumination of the rim of the horn at the feed aperture.

Referring now to FIGS. 8, 9, and 10, there is here illustrated a feed horn 39 coupled to a guide 40 through a transition 41. The born 39 is conical and has an apex at the point 42. Here a conical feed horn 39 is coupled through a tapered transition 41 to a rectangular wave guide 40. The vertex of the cone of the horn 39 is indicated at 42. Annular grooves 43 are formed in the member 39 to provide a radiation suppression means in the manner described with respect to the annulus of FIGS. 6 and 7. The modification illustrated here is useful for achieving a similar result by utilizing grooves which are closely spaced compared to a wavelength at the frequency of operation and preferably between A and /2 of a wavelength 7\ deep, or a multiple of :1 M2 deeper. The number of grooves is so selected as to provide the desired The grooves operate to produce an effective wall impedance which is at an apparent negative reactance and locally tends to lift waves which are polarized perpendicular to the wall, in addition to lifting waves polarized tangential to the wall. The effect is to lift the waves away from the wall in a manner similar to that tween the horn 32 and the guide 33. As shown here the transition flare is'indicated withrespect to the E plane.

An appropria'te flaring or matching section between the transmission line and the 'horn itself is introduced at the throat to provide proper impedance match over a broader band of frequencies. The transition section 35 has a radial surface to form a spherical transmission line, when the transverse dimension is large enough to support more than one propagating spherical wave mode. Under these conditions the phase front in the horn is spherical with phase center at the throat, in contrast with the conventional feed with a phase center at the aperture.

Referring now to FIG. 4, there is here illustrated a modification of the antenna shown in FIG. 3 wherein radiation suppression means are added to improve the E plane radiation pattern. Without the radiation suppression means, the edges of the horn are strongly illuminated in the E plane. This illumination can be reduced by means of blocking obstacles which tend to excite some of the second order spherical wave mode, the next highest symmetric mode after the dominant mode. Thus, a pair of rod-shaped elements 36 and 37 extend from the inner provided by the boundary condition of a metallic wall. This implies that both the normal and the tangential electric vectors vanish along the wall. This modification offers improved impedance matching at the throat of the horn. In addition, the frequency response is improved. Furthermore, grooves of this character are fairly readily fabricated.

The grooves extend as shown throughout the flared propagating surface. Here the grooves are of uniform rectilinear section and uniformly spaced. The distance b from the transition 41 to the edge of the first groove is chosen to be no larger than 10 at the cut-off frequency of the waveguide 40. The spacing w between grooves is chosen to be less than a half wavelength at the operating frequency. The groove width g is preferably greater than the thickness t of spacers 44; hence, t is preferably less than a quarter wavelength and g is greater than a quarter wavelength but less than a half wavelength. The spacing w is, of course, the sum of the groove width g and spacer thickness 1. The depth d may be chosen to be between a quarter wavelength and a half wavelength, or an odd multiple thereof, with satisfactory operation.

' In a primary radiator of the type illustrated in FIG. 8 actually constructed and tested the following dimensions and parameters were chosen:

Frequency of operation 4-8 kilomegacyles. Waveguide width (H plane) 1.872 inches. Waveguide height (E plane) .872i.05 inch. Dimension b .192 inch.

Dimension w .896 inch. Dimension g .704 inch. Dimension t .192 inch. Dimension d .670 inch.

It will be apparent that a wide range of operating conditions and dimensions may be selected without departing from the scope of the invention.

DESCRIPTION AND EXPLANATION OF THE RADIATOR IN FIGS. 11 AND 12 Referring now to FIGS. 11 and 12 there is here illustrated a modification of the wave translation element or primary radiator illustrated in FIGS. 8, 9 and 10. Here the radiator includes a pyramidal feed horn 45 intersect ing a rectangular waveguide 46 at a transition plane 47. The propagating surfaces of the horn 46 have elongated grooves 48 uniformly spaced throughout the flared sur- The flare of the horn may be separately adjusted The indicated faces. for the E plane relative to the H plane. E plane flare angle is 20 and H plane flare angle is 211 and scope of the invention.

It will be considered, therefore, that all those changes and modifications which fall fairly within the scope of the invention shall be a part of the invention.

What is claimed is:

1. A primary microwave antenna for use in an antenna system having a focusing wave translation means with a bounded illumination aperture and a focal point, said focal point and said illumination aperture boundary defining an illumination angle, comprising:

It is particularly useful primary wave translation means including a primary member having a bounded, plane-defining, primary wave translation aperture formed therein and tapered wave translation conductive surfaces defining with said aperture a primary wave translation aperture angle having an apex at an internal focal point, the 55 maximum dimension from said apex to said aperture minus the minimum dimension from said apex to said aperture being greater than one half wavelength of translated energy;

radiation suppression means formed in said primary wave translation surfaces, said suppression means including a plurality of transversely disposed, uniformly spaced grooves, the width of each said groove being greater than one quarter wavelength, the spacing between said grooves being less than one quarter wavelength, and the groove-to-groove spacing being approximately one half Wavelength; and

wave transmission means for coupling said member to a source of microwave energy.

2. The antenna of claim 1, wherein:

said primary wave translation aperture angle is at least 3. The antenna of claim 1, wherein:

said wave transmission means includes a wave guide centrally coupled to said primary member at a point less than one tenth of a Wavelength from the edge of the first said groove.

4. The antenna of claim 1, wherein:

said primary member is conically shaped and said grooves are annular.

5. The antenna of claim 1, wherein:

said primary member is pyramidally shaped and each said groove is quadrilaterally shaped.

6. The antenna of claim 1, wherein:

said primary Wave translation aperture angle is at least 90, said primary member is conical, said grooves are annular, and said coupling means includes a wave guide centrally coupled to said primary member at a point which is less than one tenth of one wavelength from the edge of the first said groove.

7. The antenna of claim 1, wherein:

the depth of each said groove is characterized by where n=0,1,...,k,...; )\=a wavelength of translated energy; and d=groove depth.

References Cited by the Examiner UNITED STATES PATENTS 5/1942 King 343-783 2,416,675 3/1947 Beck 343781 2,669,657 2/1954 Cutler 343-781 2,912,695 11/1959 Cutler 343-786 3,055,004 9/1962 Cutler 343-786 FOREIGN PATENTS 656,200 8/1951 Great Britain.

OTHER REFERENCES Slayton articlez Electronics-Gain Standard Horns, pp.

154, July 1955.

HERMAN KARL SAALBACH, Primary Examiner.

ELI LIEBERMAN, Examiner.

W. K. TAYLOR, Assistant Examiner, 

1. A PRIMARY MICROWAVE ANTENNA FOR USE IN AN ANTENNA SYSTEM HAVING A FOCUSING WAVE TRANSLATION MEANS WITH A BOUNDED ILLUMINATION APERTURE AND A FOCAL POINT, SAID FOCAL POINT AND SAID ILLUMINATION APERTURE BOUNDARY DEFINING AN ILLUMINATION ANGLE, COMPRISING: PRIMARY WAVE TRANSLATION MEANS INCLUDING A PRIMARY MEMBER HAVING A BOUNDED, PLANE-DEFINING, PRIMARY WAVE TRANSLATION APERTURE FORMED THEREIN AND TAPERED WAVE TRANSLATION CONDUCTIVE SURFACES DEFINING WITH SAID APERTURE A PRIMARY WAVE TRANSLATION APERTURE ANGLE HAVING AN APEX AT AN INTERNAL FOCAL POINT, THE MAXIMUM DIMENSION FROM SAID APEX TO SAID APERTURE MINUS THE MINIMUM DIMENSION FROM SAID APEX TO SAID APERTURE BEING GREATER THAN ONE HALF WAVELENGTH OF TRANSLATED ENERGY; RADIATION SUPPRESSION MEANS FORMED IN SAID PRIMARY WAVE TRANSLATION SURFACES, SAID SUPPRESSION MEANS INCLUDING A PLURALITY OF TRANSVERSELY DISPOSED, UNIFORMLY SPACED GROOVES, THE WIDTH OF EACH SAID GROOVE BEING GREATER THAN ONE QUARTER WAVELENGTH, THE SPACING BETWEEN SAID GROOVES BEING LESS THAN ONE QUARTER WAVELENGTH, AND THE GROOVE-TO-GROOVE SPACING BEING APPROXIMATELY ONE HALF WAVELENGTH; AND WAVE TRANSMISSION MEANS FOR COUPLING SAID MEMBER TO A SOURCE OF MICROWAVE ENERGY. 